The overall goal of this project is to characterize and define the relationship between microcirculatory and systemic hemodynamics and oxygenation in the setting of profound hemorrhagic shock. Hemorrhagic shock continues to be a major cause of death after trauma and despite decades of research on the initial resuscitation of the hemorrhage victim, survival has failed to significantly improve. Restoration of tissue oxygenation following hemorrhage is critical in avoiding irreversible circulatory collapse and accumulated hypoxic damage. Current treatment regimens couple restoration of tissue oxygenation with systemic hemodynamics, such as blood pressure and heart rate, but ignore the contribution of the microcirculation as an organ system. Ongoing work in our laboratory, utilizing a rodent model of prolonged hemorrhagic hypotension, is demonstrating that differences between survivors and non-survivors may be dependent on microcirculatory vasodilation to maintain oxygen delivery and consumption and limit oxygen deprivation. The mechanisms by which microvascular dynamics and tissue oxygenation are affected during hemorrhage and its treatment are not entirely understood, but may involve the important interplay between microvascular nitric oxide production and fluid resuscitation. We will utilize a novel combination of intravital microscopy of skeletal muscle with systemic hemodynamic monitoring, in the same animal. The microvascular parameters of vessel diameter, functional capillary density, red blood cell velocity, PC2 (interstitial, arteriolar, capillary, and venular) and hemoglobin oxygen saturation (arteriolar, capillary, and venular) will be compared with the systemic measures of cardiac output, oxygen delivery and consumption, mean arterial pressure, and lactate production in survivors and non-survivors. Bioimaging of nitric oxide in the microvasculature will be carried out using new fluorescent indicators. Lastly, the effects of different resuscitation fluids (normal saline, lactated Ringer's, hypertonic saline, and hetastarch) on survival and the previously mentioned microvascular and systemic parameters will be examined. A tight interplay between experiments and computational modeling of these variables and relationships will be used to interpret results and to aid in the formulation of followup experiments. Understanding how hemorrhage and its treatment modulate the microcirculation may assist in the rational development and testing of novel therapies to improve survival from hemorrhagic shock.